The invention relates to the field of dyeing of polyester films.
The energy crisis of the 1970's resulted in the development of many processes and products for energy conservation and many countries accorded priority to such efforts in parity with environmental protection. One of the means of saving energy successfully attempted by scientists has been through laminated films. U.S. Pat. Nos. 3,943,105, 4,047,889 and 4,115,054 of Julius Hermes and U.S. Pat. No. 4,399,265 of Garware et al were all directed towards this subject matter. More specifically, the Garware et al patent was directed to a new process for the manufacture of dyed film for sun control applications.
Dyed polyester films have numerous applications including solar controls films for window application, light filters in research laboratories and industrial applications, colored films for decorative applications, packaging applications and greenhouse applications.
U.S. Pat. No. 4,399,265 discloses a process for producing UV stabilized polyester films in which UV absorbers from the benzophenone and benzotriazole group are blended with polyester granules in the desired concentration, and extruded or co-extruded to obtain a UV stabilized film in various thicknesses.
U.S. Pat. No. 3,943,105 discloses a process for UV stabilization of dyed or undyed film by a solvent assisted treatment. The films are treated with UV stabilizer and carriers such as polyhydric glycols at 250 and 430.degree. F. Subsequently, the treated film is washed with water to remove excess surface solvents.
In the case of colored polyethylene terephthalate (PET) films used for solar control applications/greenhouse applications, the films are exposed to electromagnetic radiation from the sun. A high energy UV component having a wave length ranging from 200 nm to 400 nm is responsible for decomposition of organic matter, and the films must therefore be UV stabilized to prevent the destructive effects of UV radiation. Another important function of UV stabilization is to prevent UV radiation from passing through the windows.
The PET window films are generally available in laminate form, which consist of a UV stabilized colored film coated with pressure sensitive adhesive which is protected with a release liner. One of the known methods of dyeing the film is to incorporate mixtures of blue, red and yellow dyes in the polyester matrix before extrusion. The concentration of dyes to be incorporated in the polymer matrix depends upon the required shade and transmission. The dyes are mixed in the polymer matrix during polymerization, and the polymer matrix is directly fed to the extruder followed by biaxial orientation.
These films have exceptional dimensional stability against thermal aging and outdoor exposure. It is not, however, practical to produce small quantities of colored films using this method, as colors cannot be changed in the production runs.
U.S. Pat. No. 4,047,889 describes a method of waterless dyeing or solvent-assisted dyeing, which involves swelling of polyester films with high boiling polyhydric alcohols and glycols. The swelling of the polyester matrix at high temperatures is followed by a dye treatment. The dyes in contact with the polymer matrix diffuses into the film; the rate of dye diffusion depends upon the concentration of individual dyes in the bath and swelling capability of the solvent.
These films are subsequently washed with low boiling solvents such as methanol, ethanol and chlorinated low boiling solvents, which are continuously recycled in the process. This method does not generate effluent but there is very high risk of explosion and a fire hazard.
U.S. Pat. No. 4,115,054 is a continuation-in-part of the above patent, which includes the non aqueous dyeing of woven polyamide, woven polyester fibers and polyamide knitted fabrics.